Dual chamber pacemaker with adaptive atrial escape interval

ABSTRACT

A dual chamber pacemaker having an atrial escape interval which is varied on a beat-to-beat basis in response to the measured time from a ventricular event to the next atrial sensed event. Additionally, a portion of the atrial escape interval is bifurcated into a first sensing portion T1 and a second sensing portion T2 wherein atrial sense events occurring during T1 may be ignored by the pacemaker, while atrial sense events falling within T2 are used to compute a new atrial escape interval and are used to resynchronize the pacemaker and are used to inhibit the otherwise scheduled atrial pace event.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to cardiac pacemakers and more particularly to a dual chamber pacemaker which varies the pacer V-A escape interval in response to the timing of the patient's atrial activity.

2. Description of the Prior Art

The first pacemakers were asynchronous. these VOO mode pacemakers are typified by U.S. Pat. No. 3,057,456 to Greatbatch. Such pacers operate independently of intrinsic heart activity and pace the ventricle at a metronomic rate. Although such pacemakers provide a ventricular pacing rate sufficient to support life, these pacemakers can compete with intrinsic ventricular rhythms which can prove hazardous to the patient.

The VVI mode or ventricular demand pacemaker, as typified by U.S. Pat. No. 3,478,746 to Greatbatch, was introduced to reduce competitive pacing with naturally occurring ventricular rhythms. This ventricular inhibited form of pacemaker provides a ventricular sense amplifier responsive to ventricular depolarizations. The ventricular sense event recycles the V-V timer of the pacemaker. At the conclusion of the V-V time interval, the pacemaker stimulates the ventricle unless inhibited from doing so by a ventricular sense event occuring within the V-V interval.

The VAT mode pacemaker as typified by U.S. Pat. No. 3,254,596 to Keller provides an atrial sense amplifier responsive to atrial depolarizations which starts an A-V timer within the pacemaker. At the conclusion of this A-V time interval, the pacemaker generates a ventricular pacing pulse which is delivered to the ventricle. This stimulus is synchronized with the naturally occurring atrial beat. Therefore, this atrial synchronized form of ventricular pacemaker provides a prosthetic conduction pathway mimicking the natural pathway of the heart by providing a paced ventricular rhythm in response to and synchronized with the detected atrial rate. A defect of this pacing modality is the ability to compete with ectopic ventricular activity. An ectopic ventricular beat will be sensed in the atrium and treated as though it were an atrial event. This will result in the generation of a ventricular pacing stimulus a fixed A-V delay after the atrially detected ventricular event. Although such competitive pacing is considered harmless when the A-V delay of the VAT pacemaker is short, such a pacing regime can result in delivery of a ventricular pacing stimulus into the vulnerable period of the R-waves which is undesirable.

The VDD mode pacemaker, as typified by U.S. Pat. No. 3,648,707 to Greatbatch, overcomes this drawback of the VAT mode pacemaker by providing a ventricular sense amplifier responsive to ventricular depolarizations for inhibiting an otherwise scheduled ventricular pacing event by the pacemaker. In operation, the VDD pacemaker may sense atrial depolarizations through an atrial sense amplifier which starts an A-V timer. The A-V timer schedules the generation of a ventricular pacing pulse one A-V interval after the detected atrial signal. The scheduled stimulation, however, may be interrupted or inhibited by the detection of a naturally occurring ventricular event detected by the ventricular sense amplifier. In the absence of either atrial or ventricular rhythms, the VDD pacemaker will pace the ventricle at a metronomic rate. However, in the presence of a sinus atrial rhythm, the VDD pacemaker will pace the ventricle in synchrony with the detected atrial rhythm.

The DVI mode A-V sequential pacemaker, as typified by U.S. Pat. No. 3,595,242 to Berkovits, provides for stimulation in both the atrium and ventricle while providing for ventricular sensing. In this form of pacemaker, a ventricular sense event initiates a V-A timer as well as a V-V timer. At the expiration of the V-A interval, the pacemaker generates an atrial pace event, and at the expiration of the V-V interval, the pacemaker generates a ventricular pace event. If a ventricular event occurs during the V-A or V-V time, the pacemaker is inhibited and recycled so that the V-A and V-V timer restart. This bifocal form of pacemaker preserves the hemodynamic advantages of sequential atrial and ventricular contractions.

The DDI mode pacer, as typified by U.S. Pat. No. 3,747,604 to Berkovits, includes a V-A and V-V timer and further includes an atrial sense amplifier for inhibiting the generation of an atrial pacing stimulus at the conclusion of the V-A interval if a naturally occurring atrial depolarization occurs within that time interval. This DDI mode of pacemaker, unlike the VAT or VDD pacemaker, does not resynchronize or restart the pacemaker's timer upon the sensing of arial events. Therefore, it does not provide the atrial tracking response exhibited by VAT and VDD devices.

Recently, dual sense-dual pace or DDD mode pacemakers, as typified by U.S. Pat. No. 4,312,355 to Funke, have been introduced to compensate for many of the disadvantages of the preceding forms of pacemakers and for providing more efficacious therapy in the event of an intermittent block during atrial pacing. The DDD pacemaker provides an atrial sense amplifier to generate an atrial sense signal that synchronizes the pacemaker by initiating an A-V timer. The A-V delay schedules the delivery of a ventricular pace event unless a ventricular sense event occurs within the A-V interval to inhibit or cancel the scheduled ventricular pace event. This form of pacemaker also provides a ventricular sense amplifier for initiating a V-A timer in response to a ventricular sense signal which schedules the generation of an atrial stimulus at the conclusion of the V-A interval unless an atrial sense event occurs to cancel the ventricular pace event within the A-V interval.

This prior art DDD form of pacemaker is represented by the state diagram of FIG. 2. Assuming that the machine is in the atrial observation state 24 (AOT), then an atrial sense event (AS) causes state transition 11 which causes the machine to enter the A-V delay state 12 where the A-V delay timer starts to time out an atrial-ventricular escape interval. If the A-V delay timer (AVD) times out (AVTO), then state transition 14 takes place moving the machine to the ventricular pacing state (VP)16. In this state, the machine delivers a pacing stimulus to the ventricle of the heart. At the conclusion of the pacing pulse, the machine moves via transition 18 to the refractory state 20 during which time the post ventricular-atrial refractory period (PVARP) is timed out. At the conclusion of the post ventricular-atrial refractory period (PVARP), the machine moves via transition 22 to the atrial observation state (AOT). In this state, the pacer is responsive to the atrial sense amplifier as previously described. If atrial activity is not sensed, then the machine will leave the atrial observation state as indicated by state transitiion 26 whereupon entering the atrial pacing state 28, the machine generates an atrial pacing stimulus. At the conclusion of the pacing pulse reflected by transitition 30, the machine re-enters the A-V delay state 12. If a ventricular sense event occurs during this state as indicated by state transition 32, the machine re-enters the refractory state 20.

This state machine description depicts the interaction of the pacer with the heart in response to the various cardiac events which may occur during the various timing cycles of the pacer.

Each of the previously discussed pacing modalities incorporates escape intervals defined as the time period extending from a sense event to the scheduled generation of a succeeding paced event, which are fixed at a discrete value during the operation of the pacer. Modern versions of each of these prior art pacemakers provide for the remote programming of such escape interval under the direction of a physician. However, once programmed, the intervals are not altered by pacemaker events.

Pacemakers have been proposed and built which alter the escape interval of the pacemaker in response to pacemaker detected cardiac events. An example of one such pacemaker is U.S. Pat. No. 4,052,991 to Zacouto as well as U.S. Pat. No. Re. 28,341 to Gobeli. The Zacouto machine is a ventricular pacemaker with ventricular sensing which alters the V-V escape interval of the pacemaker depending upon the timing of naturally occurring ventricular events. This pacemaker measures the time interval from a ventricular pace event to a subsequent ventricular sense event and sets the escape interval of the pacemaker to that value. In operation, a Zacouto orthorhythmic pacemaker has a nomimal V-V escape interval which results in the metronomic pacing of the ventricle at that rate in the absence of detected atrial ventricular activity. When a ventricular sense event occurs, the orthorhythmic pacemaker may provide an escape interval longer than the nominal escape interval which Zacouto refers to as negative hysteresis. Alternatively the pacer may provide for a shorter escape interval than the nominal value, which Zacouto refers to as positive hysteresis. Consequently, the orthorhythmic pacemaker represents a pacemaker in which the escape interval of the pacemaker is altered on a beat-to-beat basis in response to the detection of ventricular cardiac activity.

The Gobeli pacemaker is a VVI device that exhibits positive hysteresis under the nomenclature of Zacouto since the escape interval of the pacemaker is lengthened in response to a detected ventricular sense event.

More recently, pacers have been proposed which alter the stimulation escape interval based on the evoked QT time interval. See, for example, U.S. Pat. No. 4,228,803 to Rickards and No. 4,305,396 to Wittkampf et al.

BRIEF SUMMARY OF THE INVENTION

The pacemaker of the present invention shares many structures and functions with the previously described pacemakers. However, unlike prior art DDD pacers, this pacer's V-A interval (VAI) may vary in response to detected atrial activity. Additionally this pacer alters the pacing behavior or stimulation regime of the pacer depending on the temporal relationships between an atrial sense event and the preceding ventricular event. The objective of this pacemaker is to maintain physiologic cardiac contraction patterns over a wide frequency range and to achieve this goal in the presence of a wide variety of conduction faults and arrhythmias.

The functional characteristics of this pacemaker which permit the realization of these goals include the computation of an atrial stimulation back up rate (ASBU) which adapts the intervention frequency of the pacemaker to the observed spontaneous atrial rate; sense factor (SF) which governs the pacer's atrial synchronizing behavior; and intelligent P wave treatment at upper rate (IPTUR) which governs the pacer's action in the presence of competitive rhythms.

The feature denominated SF for sense factor is very desirable in instances where the patient exhibits intermittent atrial function due to disease processes such as sick sinus syndrome or atrial flutter. In the past, such patients would not be candidates for atrial synchronized pacers such as the DDD, but would be best served by a DVI pacer. This is an unfortunate result since many patients may be denied the benefit of atrial synchronized rate responsiveness because of occasional or transient atrial dysfunction. Sense factor addresses this problem by providing the physician with a parameter which can adapt the synchronizing response of the pacer to the disease condition. In operation, the atrial observation time (AOT) of the pacer is bifurcated into two intervals denominated T1 and T2. The ratio of these two times is the physician selected sense factor (T2/T1=SF). Atrial sense events which fall within T1 are not used to resynchronize the pacer yielding a DVI-like behavior, while atrial sense events falling within T2 resynchronize the pacer yielding a DDD-like behavior. Consequently SF permits the pacer to exhibit a smooth transition between DVI and DDD behavior dependent on the temporal relationship between the atrial sense event and its preceding ventricular event. This permits the benefit of atrially based physiologic stimulation to be brought to more patients. Conceptually T1 may be regarded as a post-ventricular atrial refractory period which is adjusted automatically based on the pacer's rate. However, unlike traditional refractory periods, the atrial sense amplifier is "on" and is sensing during this time period. Note that this T1 interval is long at low rates and short at higher rates. The physician can set a large T1 period via SF programming to prohibit or prevent a pacemaker mediated tachycardia without comprising the ability of the pacer to track the faster atrial rates.

The alert reader will recognize that the benefit conferred by SF, which is the ability of the patient's atria to dictate whether the pacer will follow the DDD or DVI regime, could extract a penalty in terms of rate variability. If one recalls that DVI pacing occur at a fixed lower rate and that DDD pacing occurs at the instantaneous atrial rate, then the ability to move from DDD to DVI and back again could result in large jumps in ventricular stimulation rates. ASBU, the atrial stimulation back-up rate, addresses this problem. This feature is described in excruciating detail later. For now it should be considered as a "flywheel" or the pacer's escape interval. The computation of the pacer's atrial escape interval contains "inertial" terms which ensure that the rate accelerates in a physiological sensible manner.

We now move to the behavior of the pacer at high rates. Once again the matter is described later in unrelenting detail, however, the IPTUR feature should be examined briefly to understand its relation to the schema. If a sequence of atrial sense events have fallen in T2, then the pacer's ASBU has been shortened to approximate the naturally observed Q-P or V-A time. When the pacer's rate and the heart's rate are similar, one runs the risk of competitive pacing. Competitive pacing occurs when a natural cardiac depolarization and a pacing stimulus occur together close in time. Let us assume that ASBU has scheduled an atrial stimulus 150 ms hence, and at present we sense a naturally occurring atrial event which has occurred in T1. IPTUR dictates that we cancel the scheduled atrial pace event even though it has landed in T1 and would at lower rates be ignored (DVI-like behavior). IPTUR, in essence, sets a window of inhibition which overrides the SF dictated response at higher rates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a timing diagram showing the relationships between intervals used to describe the invention.

FIG. 2 is a state diagram of a prior art DDD pacer.

FIGS. 3, 4 and 5 are hypothetical schematic timing diagrams depicting the pacer timing intervals and the simultaneous electrocardiographic recordings which show the interaction between the pacer and the heart.

FIG. 6 is a state diagram depicting the pacer of the present invention.

FIG. 7 is a functional block diagram of the pacemaker of the present invention.

DESCRIPTION OF THE INVENTION

Throughout the description, reference will be made to terms defined as follows:

VE is a ventricular event, either sense or pace;

VP is a ventricular pace event generated by the ventricular pulse generator.

VS is a venytricular sense event generated by the ventricular sense amplifier.

AP is an atrial pace event generated by the atrial pulse generator.

AS is an atrial sense event generated by the atrial sense amplifier.

[VE→AP]_(n) is the time interval from a ventricular event to the next scheduled AP event for the nth pacing cycle. This corresponds to the V-A escape interval of the pacer.

ASBU is the atrial stimulation back-up rate which corresponds to the V-A escape interval of the pacer. It comprises the arithmetic summation of two time intervals and is computed by the pacer in real time. [PVARP+AOT].

PVARP is the post ventricular atrial refractory period. The value is programmable. The total atrial refractory period is equal to the arithmetic sum of PVARP and the A-V delay interval of the pacer.

AOT is the atrial observation time. This period extends from the end of the PVARP to the end of the ASBU. [ASBU-PVARP]=AOT.

T1 is the first portion of the AOT.

T2 is the second portion of the AOT.

SF is the sense factor of the pacer and is the ratio of T2/T1. The value of SF is programmable and should vary from 10.0>SF>0.1.

K1 is a programmable constant having a nominal value of 12 ms which is added to the nth value of [VE→AP] to generate the n+1 value of [VE→AP] or ASBU when an AS event occurs in T2.

K2 is a programmable constant having a nominal value of 50 ms which is added to the nth value of [VE→AP] to generate the n+1 value of [VE→AP] or ASBU when no AS events occur in the AOT.

K3 is a programmable constant having a nominal value of 150 ms. This time interval overlaps the last protion of T2 and ends with the AP event. Any AS event within the K3 portion of T2 is used to compute the next ASBU and is used to inhibit the impeding AP event. This provides protection from competitive pacing in the atrium.

As shown in connection with FIG. 1, the "V-A" or atrial escape interval of the pacemaker may be varied on a beat-to-beat basis. The atrial escape or VA interval is defined as the time from a ventricular event (VE) labeled 1 to the next atrial pace event (AP) olabeled 2 and may be expressed [VE→AP)]_(n). This time interval [VE→AP] may be shortened for a subsequent pacing cycle [VE→AP]_(n+1) if an atrial event occurs within this time interval during a current pacing cycle. Similarly, the subsequent VA, atrial escape interval [VE→AP]_(n+1) of the pacemaker may be extended if no atrial activity is detected within the current interval. The atrial escape interval [VE→AP]_(n) is referred to as the atrial stimulation back-up interval. It may be recomputed on each beat, and it is abbreviated ASBU and labeled 3 in the figure. ASBU will vary between a preset maximum value and a preset minimum value. ASBU reflects the interval from a ventricular event to the next scheduled atrial pace event for any given pacing cycle.

In a DDD mode pacemaker, the atrial refractory period is extended beyond ventricular events. Consequently, the pacer exhibits a post ventricular atrial refractory period (PVARP) labeled 4 in the figure, which renders the pacer refractory or insensitive to atrial depolarizations occurring within this fixed time after the ventricular event, depicted by atrial sense event 8 in the figure. In most DDD pacers, this PVARP is a programmed parameter. In a conventional DDD pacer, the conclusion of the PVARP starts a time window during which the pacer can sense atrial events. For our purposes, we define the time period starting with the end of the atrial refractory period to the time out of the ASBU interval as the atrial observation time, abbreviated AOT and labeled 5 in the figure.

In the present invention, the AOT is bifurcated into a first sensing period T1 labeled 6 and a second sensing period T2 labeled 7 where AOT=T1+T2. The ratio of time intervals T2/T1 is named SF for sense factor. The sense factor may range from 0.1 10.0 and is a physician programmable parameter, 10.0>SF>0.1.

The maximum value of ASBU is defined by physician selection of the lower pacing rate for the pacemaker while the minimum value for ASBU is comparable to the duration of the post ventricular atrial refractory period which is defined by the physician when he selects the atrial refractory period.

In operation, ASBU is shortened when an atrial sense event occurs in T2 depicted by atrial sense event 9 in FIG. 1. In this instance, the pacer shortens the atrial escape interval so that it just exceeds the observed atrial rate by a small amount. In this instance, the value of ASBU is set equal to the time period from the ventricular event ot the atrial sense event with the addition of a small increment of positive hysteresis K1, ASBU_(n+1) =[VE→AS]_(n+) K1, where the value of K1 is on the order of 12 ms and is programmable.

Consequently, an atrial sense event in the T2 portion of the atrial observation time (AOT) can accelerate the pacer through the ASBU calculation. Additionally this atrial sense event during T2 resynchronizes the pacer by starting the A-V delay timer and also inhibits the otherwise scheduled atrial pace event.

Clearly atrial sense events can also occur during the T1 portion of the atrial observation time as depicted in the figure by atrial sense event 10. The pacer does not use atrial sense events during T1 to update the ASBU value. Atrial events sensed during T1 may or may not be used to inhibit the scheduled atrial pace event depending on the temporal proximity between the atrial sense and the scheduled atrial pace events. If the time from the As event to the AP event is less than a constant K3, then the scheduled AP even is cancelled [AS→AP]<K3, and the pacer exhibits an atrial inhibited response. If, however, the time from the As event to the AP exceeds K3, then the pacer will not cancel the AP event and will not exhibit an atrial inhibited behavior.

In the alternative, a cardiac cycle may transpire wherein no atrial sense event occurs. In this case, the value of ASBU is lengthened gradually. In this instance the n+1 value of ASBU is set equal to the preceeding n value with the addition ofa fallback interval K2; ASBU_(n+1) =ASBU_(n) +K2. The value of K2 is in the order of 50 ms and is programmable.

In conclusion, it should be observed that ASBU may be shortened or accelerated by rapidly recurring atrial activity, and ASBU will be lengthened by either the absence of atrial activity or the occurrence of atrial activity early in the cardiac cycle.

FIGS. 3, 4 and 5 shown schematic representations for a pacemaker according to the present invention. In each figure, a schematic EKG trace is shown in conjunction with the various pacemaker time intervals to depict the interaction of the pacemaker with the heart.

Turning to FIG. 3, cardiac pacing cycle A shows the sequential stimulation of both chambers of the patient's heart at the programmed lower rate limit (LR). The electrogram begins with a spontaneously occurring R-wave at 49 which gives rise to a ventricular sense event at 50, initiating the post ventricular atrial refractory period of the pacemaker shown at 52. At the conclusion of the atrial refractory period, the pacemaker enters the atrial observation time. The first portion of the atrial observation time is labeled T1 in the diagram and begins at 54. At the conclusion of the first portion of the observation time, the second portion of the observation time begins at 56. As shown in FIG. 3, no atrial sense events occur within the atrial observation time. Therefore, at the conclusion of this time period, the pacemaker emits an atrial pacing pulse at 58, depicted as the pacing artifact 59 preceding atrial complex or P-wave 61.

This time interval from a ventricular event 50 to the subsequent generation of an atrial pacing event 58 is referred to as the atrial escape interval of the pacemaker and is referred to as the atrial stimulation back-up interval or ASBU. Complex A shows a pacemaker which has a programmed post ventricular-atrial refractory period of 200 ms with a V-A excape interval programmed to 600 ms. The sense factor of this pacemaker has been set at 1 thus bifurcating the atrial observation time which is defined as the time period from the conclusion of the atrial refractory period to the generation of the atrial pacing event 58 into two equal time intervals T1 and T2. Complex A represents the maximum value for ASBU for the programmed settings enumerated above.

After the generation of the atrial pacing event 58, the A-V delay timer of the pacemaker begins timing out the A-V interval shown as 60 in FIG. 3. At the conclusion of this interval, a ventricular pacing event 62 is generated since no ventricular sensed activity was detected by the pacemaker within the A-V interval. The ventricular pace event 62 gives rise to the pacing artifact 63 shown on the schematic EKG prior to the stimulated R-wave 64.

The ventricular event 62 once again starts the post ventricular atrial refractory period timer at 66. At the conclusion of the atrial refractory period, the atrial observation time begins at 68. During the first portion T1 of the atrial observation time, an atrial sense event occurs at 70. This is shown in the schematic EKG as P-wave 72. This P-wave, since it falls within T1, does not give rise to a new value for ASBU nor does not inhibit the scheduled atrial pace event for the cardiac cycle shown in B in FIG. 3, nor does it initiate the A-V timer. In this particular instance, the pacemaker treats atrial event 70 as if it fell within an atrial refractory period.

Once again, at the conclusion of the atrial observation time at 72. the pacemaker generates an atrial pace event 74 giving rise to a pacing artifact shown on the schematic EKG at 73 preceding P-wave 75.

The atrial pace event at 74 initiates the A-V delay timer resulting in the generation of a ventricular pacing pulse 76 at the conclusion of the A-V delay time and shown on the schematic EKG as pacing artifact 77 preceding the stimulated R-wave 78. At this point, the pacemaker enters the cardiac cycle C in FIG. 3. The computed atrial stimulation back-up rate for this cardiac cycle is 600 ms based upon the information shown within the preceding cardiac cycles A and B. During this cardiac cycle, however, an atrial sense event 80 occurs during the latter portion of the atrial observation time falling within T2. The atrial sense event 80 truncates the remaining portion of T2 and starts the A-V delay timer at 84. Since no naturally conducted R-wave follows P-wave 86, the A-V delay timer times out at 88 generating the ventricular pacing artifact 87 preceding ventricular complex 89.

In pacing cycle D, the occurrence of the atrial sense event 80 during the atrial observation time has accelerated the pacemaker by shortening the atrial escape interval.

ASBU for cardiac cycle D is 510 ms. This value is computed by adding a small hysteresis increment of 10 ms to the observed [VP→AS] time interval of 500 ms of cardiac cycle C. The sense factor (SF) remains constant at 1 thus bifurcating the 310 ms atrial observation time into equal 155 ms sense windows T1, T2.

During cardiac cycle D no atrial events are detected. Therefore, the pacemaker provides the atrial pacing event 90 at the conclusion of the atrial observation time giving rise to the pacing artifact 91 preceding the provoked P-wave 92 on the EKG. At the conclusion of the A-V delay, the pacemaker generates a ventricula4 pace event 94 giving rise to the pacing artifact 95 preceding stimulated R-wave 97.

Since there has been no detected atrial activity during cardiac cycle D, the value of ASBU is incremented for cardiac cycle E. A fallback increment K2 is added to the value of ASBU. The fallback increment K2 may vary between 10 and 100 ms and has a nominal value of 50 ms giving rise to the ASBU for cardiac cycle E of 560 ms.

Although the fallback increment may be made a constant, it is also contemplated that this value may be logically coupled to the SF value selected by the physician. In general, large values of SF would correspond to large values of K2, and low SF values would map to small values of K2.

With respect to FIG. 3, the computation of the atrial stimulation back-up interval has been shown. The pacemaker's atrial escape interval was accelerated by atrial event 80 and was extended in the absence of atrial activity as shown in connection with cardiac cycle E.

In operation, the value of ASBU may vary from a maximum imposed by the lower rate programmed by the physician as depicted in cardiac cycles A and B up to a minimum value dictated by the duration of the PVARP.

The operation of the pacemaker at high atrial rates and at various sense factors is shown in connection with FIGS. 4 and 5.

Turning to FIG. 4, the lower rate of the pacemaker has remained programmed to the 600 ms value, and the PVARP has remained at 200 ms. In the figure, however, the sense factor has been changed from 1 to 0.25, yielding a larger T1 portion of the atrial observation time.

In the figure, pacing cycle J begins with spontaneous R-wave 400 which gives rise to ventricular sense event 402 initiating the post ventricular atrial refractory period at 404. At the conclusion of the PVARP, the atrial observation time begins. In this example, the SF is 0.33.

The absence of atrial activity during the AOT results in the scheduled stimulation of the atria at 410, the conclusion of the atrial observation time is depicted by pacing artifact 412 preceding P-wave 414. This atrial pace event 410 starts the A-V delay time of the pacer at 418. At the conclusion of the A-V delay time, a ventricular pace event 420 is generated as illustrated by pacing artifact 422 preceding R-wave 424.

The events of pacing cycle M are identical to pacing cycle J. Pacing cycles K and L are similar to pacing cycles J and M except that atrial activity occurs during cycles K and L.

In pacing cycle K, the atrial sense event occurs at 426 within the T1 portion 428 of the atrial observation time. The pacer computes the time interval remaining until the next scheduled atrial pace event 430. This time interval [AS→AP] is compared with a physician programmable constant K3. Then the pacer effectively ignores the atrial sense event in the sense that the machine remains in the atrial observation state. When the computed interval is shorter than K3, the pacer will inhibit the scheduled atrial pace event as more fully described in connection with pacing cycle H.

Note that ASBU for each pacing cycle in FIG. 4 has remained constant. This occurs in the example of FIG. 4 because atrial events in T1 do not accelerate the pacer by shortening ASBU, and the increment K2 which may be added to ASBU after each cycle cannot extend the value of ASBU below the lower programmed state.

FIG. 5 is similar to FIG. 4 in that all programmable values are the same save sense factor which has a value of 0.25 in FIG. 5. Also, the occurrence of atrial event being the same between FIGS. 4 and 5.

Turning to FIG. 5, note that due to the change in SF, the atrial sense event 500 has occurred during the first few milliseconds of the T2 portion 502 of the atrial observation time.

In response to As 500, the pacer enters the A-V delay state wherein the programmed A-V delay is timed out. At the conclusion of this A-V delay 504, a ventricular pace event 506 is generated completing pacing cycle F.

ASBU for pacing cycle G is calculated based on the occurrence of AS 500. The pacer sets ASBU for pacing cycle G equal to the observed time interval from the ventricular event 501 to the atrial sense event 500 of 310 ms with the addition of an increment K1 of 10 ms. Therefore, the value of ASBU for pacing cycle G is 320 ms with an AOT value of 120 ms. Since no atrial sense events occur during pacing cycle G, the ASBU for pacing cycle H is incremented by K2=50 ms yielding an ASBU of 370 ms; AOT=170 ms; T1=42.5 ms and T2=127.5 ms.

During T1 of cycle H, an atrial sense event occurs 508. The pacer once again computes the time to the next atrial pace event which in this case is approximately 120 ms. [AS→AP]>130 ms. In the example, we assume a value of K3 of 150 ms and note that [AS→AP] 150 ms. This illustrates that while operating near the upper rate limit the duration of T1 and T2 may be quite short which reduces the likelihood that all P-waves will be sensed within T2. In the illustration, the naturally occurring atrial event 508 occurs so close to the scheduled atrial pace event that there is a likelihood that the scheduled AP event would fall within the repolarization or vulnerable time of the atrial and thus compete with the natural atrial rhythm. To avoid this, possibly the pacer inhibits atrial pace events which would occur less than a preset time, i.e. K3 prior to the atrial stimulation time.

The state machine depicted in FIG. 6 describes the differences between the pacer according to the present invention and the pacer disclosed in U.S. Pat. No. 4,312,355.

At the conclusion of the post ventricular atrial refractory, the pacer enters the atrial observation state 200 via state transition atrial refractory. The pacer enters the atrial observation state 200 via state transition 202. The atrial observation time (AOT) is bifurcated by the sense factor (SF), and the total duration of the T1 and T2 segments depends on the SF as well as the observed atrial sensing rate. If an atrial sense event occurs during T1 and the computed time period from the atrial sense event AS to the end of the AOT exceeds the value of K3, then the pacer remains in AOT state depicted by loop 204. In this situation, the machine effectively ignores the detected atrial event. If, however, the AS event in T1 gives rise to a value of [AS→AP] which is smaller than K3, then the pacer will inhibit the otherwise scheduled atrial pace event AP and move to the A-V delay state 206 (AVD) at the conclusion of the atrial observation time (AOT). This is shown by state transition 205.

Another path for leaving the AOT state 200 is shown by state transition 208. In this instance, there has been no atrial sense event AS, and the atrial observation time has expired at the end of T2. The machine then enters the atrial pace state 209 where the pacer generates an atrial stimulus on the atrial lead.

As illustrated in FIG. 7, the machine herein described has been reduced to practice through the use of a commercially available Commodore PET personal computer 600 coupled to conventional pacer atrial sense amplifier 602, ventricular sense amplifier 604, atrial output stimulus generator 606, and ventricular output stimulus generator 608. All timing functions are performed by computer 600. An annotated software listing written in 6502 ASSEMBLER CODE is reproduced as follows:

    __________________________________________________________________________     100                                                                               rem *** ddd-pacemaker with gliding atrial                                      refractory ***                                                              105                                                                               rem *** vsap and intelligent p wave treatment                                  (ipt) ***                                                                   110                                                                               rem *** basref15+masref15 h.d.funke aug. 14,                                   1983 ***                                                                    115                                                                               rem *** ipt-time 20494,x * ipt off 20529,0 * ipt on                            20529,251 ***                                                               120                                                                               rem *** d00 20020,0 * 20029,0 * 20070,0 * 20079,0 *                            20202,0 ***                                                                 125                                                                               poke59468,14:poke59459,67                                                   130                                                                               print"   Please wait, I'm loading":                                            print:print                                                                 135                                                                               print"   the machine program!"                                              140                                                                               fori=39645to40197:pokei-19645,peek(i):nexti                                 145                                                                               1r$="70":av$="180":hy$="25":sf$=".3":hr$="120":                                wa$=".5":wv$=".5":mo$="1"                                                   150                                                                               print"   *** DDD-Autoref-Pacemaker ***":print:                                 print:goto160                                                               155                                                                               print "  "                                                                  160                                                                               bl=40:re=150:cali=115:w7=30                                                 165                                                                               print"  Lo Rate ( 50-120 PPM) ?";tab(32);1r$;                               170                                                                               ifr=Othenprint" ":goto180                                                   175                                                                               print"  ";tab(30);:inputlr$:print                                           180                                                                               1r=val (1r$):iflr<50orlr>120thenprint"  ":gotto165                          185                                                                               print"  AV Time ( 20-250 ms) ?";tab(32);av$;                                190                                                                               ifr=Othenprint"   ":goto200                                                 195                                                                               print"  ":tab(30);:inputav$:print                                           200                                                                               av=val (av$):ifav<20orav>250thenprint"  ":goto165                           205                                                                               print" AV Hyst ( 00-100 ms) ?";tab(32);hy$;                                 210                                                                               ifr=Othenprint"  ":goto220                                                  215                                                                               print"  ";tab(30);:inputhy$:print                                           220                                                                               hy=val(hy$):ifhy>av-20orhy>100thenprint"  ":goto205                         225                                                                               print" Sense Fact ( 0.1-1.0 ) ?";tab(32);sf$;                               230                                                                               ifr=Othenprint"  ":goto240                                                  235                                                                               print"  ";tab(30);:inputsf$:print                                           240                                                                               sf=val(sf$):ifsf<.1orsf>1thenprint"  ":goto225                              245                                                                               mr=int(60000/(av+re+20))                                                    250                                                                               print"  Hi Rate (";int(1r+1);:if1r+>99.9thenprint                              "  ";                                                                       255                                                                               ifint(1r+1)<100thenpoke33745,32                                             260                                                                               print"- ";int(mr);tab(23);"PPM) ?";tab(32);hr$;                             265                                                                               ifr=Othenprint"  ":goto275                                                  270                                                                               print"  ";tab(30);:inputhr$:print                                           275                                                                               hr=val(hr$):ifhr<1r+1orhr>mrthenprint"  ":goto250                           280                                                                               print"  PWA  ( 00-2.0 ms) ?";tab(32);wa$;                                   285                                                                               ifr=Othenprint"  ":goto295                                                  290                                                                               print"  ";tab(30);:inputwa$:print                                           295                                                                               wa=val(wa$):ifwa>2thenprint"  ":goto280                                     300                                                                               print"  PWV  ( 00-2.0 ms) ?";tab(32);wv$;                                   305                                                                               ifr=Othenprint"  ":goto315                                                  310                                                                               print"  ";tab(30);:inputwv$:print                                           315                                                                               wv=val(wv$):ifwv>2thenprint"  ":goto300                                     320                                                                               print"  DDD(1), DVI(2), VVI(3) ?";tab(32);mo$;                              325                                                                               ifr=Othenprint"  ":goto335                                                  330                                                                               print"  ";tab(30);:inputmo$:print                                           335                                                                               mo=val(mo$):ifmo<>1andmo<>2andmo<>3thenprint"  ":                              goto320                                                                     340                                                                               print:print"  New entries: press any key."                                  345                                                                               print"  New start: run 145":r=1                                             350                                                                               rem *** 1r, av, hy, sf, hr, bl, re, wa, wv, cali,                              w7 ***                                                                      355                                                                               av=int(av/4):hy=int(hy/4):bl=int(bl/4):re=int(re/4):                           w7=int(w7/4)                                                                360                                                                               wa=int(wa*115):wv=int(wv*115)                                               365                                                                               poke20128,bl:poke20251,re:poke20106,wa:poke20229,wv                         370                                                                               poke20010,ca:poke20060,ca:poke20133,ca                                      375                                                                               poke20183,ca:poke20256,ca:poke20306,ca                                      380                                                                               poke20104,254:ifwa=Othenpoke20104,255:poke20106,1                           385                                                                               poke20227,253:ifwv=Othenpoke20227,255:poke20229,1                           390                                                                               ifmo=1thenpoke20070,251:poke20104,254:ifwa=Othenpoke                           20104,255                                                                   395                                                                               ifmo=2thenpoke20070,0:poke20104,254:ifwa=Othenpoke                             20104,255                                                                   400                                                                               ifmo=3thenpoke20070,0:poke20104,255                                         405                                                                               w4=av-bl-w7:w6=av-hy-w7:ws=int(15000/1r)-av-re:rem                             ws=w1+w2                                                                    410                                                                               w1=int(ws/(sf+1)):w2=ws-w1:tm=int(15000/hr)-re-w6-w7                        415                                                                               rem *** w1, w2, b1, w4, re, w6, tm, w7 ***                                  420                                                                               ifw1<1thenw1=1                                                              425                                                                               ifw2<1thenw2=1                                                              430                                                                               ifw4<1thenw4=1                                                              435                                                                               ifw6<1thenw6=1                                                              440                                                                               iftm<1thentm=1                                                              445                                                                               poke20370,tm:poke20390,tm:poke20410,tm:poke20429,w7                         450                                                                               poke20353,w1:poke20005,tm:poke20055,w2:poke20178,w4:                           poke 20301,w6                                                               455                                                                               sys20004:geta$:ifa$<>""thengoto155                                          460                                                                               poke20055,int(peek(20005)*sf):ifpeek(20005)<                                   tmthenpoke20005,tm                                                          465                                                                               goto455                                                                     ready.                                                                         20000                                                                              rts                                                                        20001                                                                              nop                                                                        20002                                                                              brk                                                                        20003                                                                              brk                                                                        20004                                                                              1da                                                                               50                                                                      20006                                                                              stam                                                                              20002                                                                   20009                                                                              1da                                                                               115                                                                     20011                                                                              stam                                                                              20003        WAIT P.R.                                                  20014                                                                              1dam                                                                              59471                                                                   20017                                                                              ora                                                                               251                                                                     20019                                                                              cmp                                                                               251                                                                     20021                                                                              beq                                                                               20045                                                                   20023                                                                              1dam                                                                              59471                                                                   20026                                                                              ora                                                                               247                                                                     20028                                                                              cmp                                                                               247                                                                     20030                                                                              beq                                                                               20048                                                                   20032                                                                              decm                                                                              20003                                                                   20035                                                                              bne                                                                               20014                                                                   20037                                                                              decm                                                                              20002                                                                   20040                                                                              bne                                                                               20009                                                                   20042                                                                              jmp                                                                               20528                                                                   20045                                                                              jmp                                                                               20476                                                                   20048                                                                              jmp                                                                               20250                                                                   refract.                                                                       20051                                                                              nop                                                                        20052                                                                              orap                                                                              87                                                                      20054                                                                              1da                                                                               14                                                                      20056                                                                              stam                                                                              20052                                                                   20059                                                                              1da                                                                               115                                                                     20061                                                                              stam                                                                              20053                                                                   20064                                                                              1dam                                                                              59471                                                                   20067                                                                              ora                                                                               251                                                                     20069                                                                              cmp                                                                               251          WAIT P.R.                                                  20071                                                                              beq                                                                               20095                                                                   20073                                                                              1dam                                                                              59471                                                                   20076                                                                              ora                                                                               247                                                                     20078                                                                              cmp                                                                               247                                                                     20080                                                                              beq                                                                               20098                                                                   20082                                                                              decm                                                                              20053                                                                   20085                                                                              bne                                                                               20064                                                                   20087                                                                              decm                                                                              20052                                                                   20090                                                                              bne                                                                               20059                                                                   20092                                                                              jmp                                                                               20102                                                                   20095                                                                              jmp                                                                               20365                                                                   atrial stim. rate back-up (ASBU)                                               20098                                                                              jmp                                                                               20250                                                                   refract                                                                        20101                                                                              nop                                                                        20102                                                                              sei                                                                        20103                                                                              1da                                                                               254                                                                     20105                                                                              1dx                                                                               57                                                                      20107                                                                              stam                                                                              59471                                                                   20110                                                                              dex                                                                        20111                                                                              nop                                                                        20112                                                                              nop    STIM A                                                              20113                                                                              nbe                                                                               20110                                                                   20115                                                                              1dx                                                                               255                                                                     20117                                                                              stym                                                                              59471                                                                   20120                                                                              c1i                                                                        20121                                                                              jmp                                                                               20348                                                                   SLOW DOWN                                                                      20124                                                                              nop                                                                        20125                                                                              brk                                                                        20126                                                                              brk                                                                        20127                                                                              1da                                                                               10                                                                      20129                                                                              stam                                                                              20125                                                                   20132                                                                              1da                                                                               115                                                                     20134                                                                              stam                                                                              20126                                                                   20137                                                                              1dam                                                                              59471                                                                   20140                                                                              ora                                                                               251          WAIT BLANKING                                              20142                                                                              cmp                                                                               0                                                                       20144                                                                              beq                                                                               20168                                                                   20146                                                                              1dam                                                                              59471                                                                   20149                                                                              ora                                                                               247                                                                     20151                                                                              cmp                                                                               0                                                                       20153                                                                              beq                                                                               20171                                                                   20155                                                                              decm                                                                              20126                                                                   20158                                                                              bne                                                                               20137                                                                   20160                                                                              decm                                                                              20125                                                                   20163                                                                              bne                                                                               20132                                                                   20165                                                                              jmp                                                                               20177                                                                   20168                                                                              jmp                                                                               0                                                                       20171                                                                              jmp                                                                               0                                                                       20174                                                                              nop                                                                        20175                                                                              brk                                                                        20176                                                                              brk                                                                        20177                                                                              1da                                                                               28                                                                      20179                                                                              stam                                                                              20175                                                                   20182                                                                              1da                                                                               115                                                                     20184                                                                              stam                                                                              20176                                                                   20187                                                                              1dam                                                                              59471                                                                   20190                                                                              ora                                                                               251                                                                     20192                                                                              cmp                                                                               0            WAIT                                                       20194                                                                              beq                                                                               20218        REFRACT.                                                   20196                                                                              1dam                                                                              59471                                                                   20199                                                                              ora                                                                               247                                                                     20201                                                                              cmp                                                                               247                                                                     20203                                                                              beq                                                                               20221                                                                   20205                                                                              decm                                                                              20176                                                                   20208                                                                              bne                                                                               20187                                                                   20210                                                                              decm                                                                              20175                                                                   20213                                                                              bne                                                                               20182                                                                   20215                                                                              jmp                                                                               20428                                                                   Wait R for R-inhib??? without VSAP                                             20218                                                                              jmp                                                                               0                                                                       20221                                                                              jmp                                                                               20385                                                                   VSAP                                                                           20224                                                                              nop                                                                        20225                                                                              sei                                                                        20226                                                                              1da                                                                               253                                                                     20228                                                                              1dx                                                                               57                                                                      20230                                                                              stam                                                                              59471        STIM. V                                                    20233                                                                              dex                                                                        20234                                                                              nop                                                                        20235                                                                              nop                                                                        20236                                                                              bne                                                                               20233                                                                   20238                                                                              1dx                                                                               255                                                                     20240                                                                              stxm                                                                              59471                                                                   20243                                                                              cli                                                                        20244                                                                              jmp                                                                               20250                                                                   Refract.                                                                       20247                                                                              nop                                                                        20248                                                                              brk                                                                        20249                                                                              brk                                                                        20250                                                                              1da                                                                               37                                                                      20252                                                                              stam                                                                              20248                                                                   20255                                                                              1da                                                                               115                                                                     20257                                                                              stam                                                                              20249                                                                   20260                                                                              1dam                                                                              59471        WAIT REFRACT.                                              20263                                                                              ora                                                                               251                                                                     20265                                                                              cmp                                                                               0                                                                       20267                                                                              beq                                                                               20291                                                                   20269                                                                              1dam                                                                              59471                                                                   20272                                                                              ora                                                                               247                                                                     20274                                                                              cmp                                                                               0                                                                       20276                                                                              beq                                                                               20294                                                                   20278                                                                              decm                                                                              20249                                                                   20281                                                                              bne                                                                               20260                                                                   20283                                                                              decm                                                                              20248                                                                   20286                                                                              bne                                                                               20255                                                                   20288                                                                              jmp                                                                               20538                                                                   20291                                                                              jmp                                                                               0                                                                       20294                                                                              jmp                                                                               0                                                                       20297                                                                              nop                                                                        20298                                                                              brk                                                                        20299                                                                              brk                                                                        20300                                                                              1da                                                                               32                                                                      20302                                                                              stam                                                                              20298                                                                   20305                                                                              1da                                                                               115                                                                     20307                                                                              stam                                                                              20299                                                                   20310                                                                              1dam                                                                              59471                                                                   20313                                                                              ora                                                                               251 WAIT R                                                              20315                                                                              cmp                                                                               0   (after atrial sense)                                                20317                                                                              beq                                                                               20341                                                                              VSAP - DECIMAL?                                                     20319                                                                              1dam                                                                              59471                                                                   20322                                                                              ora                                                                               247                                                                     20324                                                                              cmp                                                                               247                                                                     20326                                                                              beq                                                                               20344                                                                   20328                                                                              decm                                                                              20299                                                                   20331                                                                              bne                                                                               20310                                                                   20333                                                                              decm                                                                              20298                                                                   20336                                                                              bne                                                                               20305                                                                   20338                                                                              jmp                                                                               20428                                                                   Wait R for R-in???? witout VSAP                                                20341                                                                              jmp                                                                               0                                                                       20344                                                                              jmp                                                                               20405                                                                   VSAP                                                                           20347                                                                              nop                                                                        20348                                                                              clc                                                                        20349                                                                              1dam                                                                              20005                                                                   20352                                                                              cmp                                                                               101                                                                     Lowest rate needed         SLOW DOWN                                           20354                                                                              bcs                                                                               20361                                                                   20356                                                                              adc                                                                               2                                                                       Slow down by 2×4=8ms                                                         AFTER                                                                      20358                                                                              stam                                                                              20005                                                                   20361                                                                              jmp                                                                               20127                                                                   Blanking                                                                           ATRIAL STIM                                                                20364                                                                              nop                                                                        20365                                                                              sec                                                                        20366                                                                              1dam                                                                              20005                                                                   20369                                                                              cmp                                                                               49                                                                      20371                                                                              bcc                                                                               20381                                                                   20373                                                                              sbcm                                                                              20052                                                                   This time from wait P.R.                                                                  elapse                                                              20376                                                                              adc                                                                               5                                                                       Soothing rate changes                                                              ASBU                                                                       20378                                                                              stam                                                                              20005                                                                   Store new time back                                                            20381                                                                              jmp                                                                               20300                                                                   Wait R                                                                         20384                                                                              tax                                                                        20385                                                                              sec                                                                        20386                                                                              1dam                                                                              20005                                                                   20389                                                                              cmp                                                                               49                                                                      Control for already existing                                                              maximal rate                                                        20391                                                                              bcc                                                                               20401               VSAP                                                20393                                                                              sbcm                                                                              20175                                                                   This time from wait R could                                                        AFTER                                                                                 not elapse because of early                                                                    ATRIAL                                                         R.              STIM                                                20396                                                                              sbc                                                                               4                                                                       20398                                                                              stam                                                                              20005                                                                   Time for acceleration stored back                                              20401                                                                              jmp                                                                               20250                                                                   Refract.                                                                       20404                                                                              tax                                                                        20405                                                                              sec                                                                        20406                                                                              1dam                                                                              20005                                                                   20409                                                                              cmp                                                                               49                                                                      Max rate checking required?                                                    20411                                                                              bcc                                                                               20421               VSAP                                                20413                                                                              sbcm                                                                              20298                                                                   This time could not elapse                                                         AFTER                                                                                 because of early R.                                                                            ATRIAL                                              20416                                                                              sbc                                                                               4                   SENSE                                               20418                                                                              stam                                                                              20005                                                                   Later time stored back                                                         20421                                                                              jmp                                                                               20250                                                                   Refract.                                                                       20424                                                                              tax                                                                        20425                                                                              nop                                                                        20426                                                                              brk                                                                        20427                                                                              brk                                                                        20428                                                                              1da                                                                               7                                                                       20430                                                                              stam                                                                              20426                                                                   20433                                                                              1da                                                                               125 WAIT R                                                              20435                                                                              stam                                                                              20427                                                                              (for R-inhibition)                                                  20438                                                                              1dam                                                                              59471                                                                              (if R, no VSAP)                                                     20441                                                                              ora                                                                               251                                                                     20443                                                                              cmp                                                                               0                                                                       20445                                                                              beq                                                                               20469                                                                   20447                                                                              1dam                                                                              59471                                                                   20450                                                                              ora                                                                               247                                                                     20452                                                                              cmp                                                                               247                                                                     20454                                                                              beq                                                                               20472                                                                   20456                                                                              decm                                                                              20427                                                                   20459                                                                              bne                                                                               20438                                                                   20461                                                                              decm                                                                              20426                                                                   20464                                                                              bne                                                                               20433                                                                   20466                                                                              jmp                                                                               20225                                                                   Stim R.                                                                        20469                                                                              jmp                                                                               0                                                                       20472                                                                              jmp                                                                               20250                                                                   Refractory                                                                     20475                                                                              tax                                                                        20476                                                                              tax                                                                        20477                                                                              tax                                                                        20478                                                                              tax                                                                        20479                                                                              tax                                                                        20480                                                                              1da                                                                        20482                                                                              stam                                                                              20020                                                                   20485                                                                              nop                                                                        20486                                                                              1dam                                                                              20002                                                                   20489                                                                              adcm                                                                              20055                                                                   20492                                                                              nop                                                                        20493                                                                              cmp                                                                               60                                                                      1PT-Time                                                                       20495                                                                              bcs                                                                               20513                                                                   20497                                                                              nop                                                                        20498                                                                              1da                                                                               79                                                                      20500                                                                              stam                                                                              20094                                                                   20503                                                                              1da                                                                               124                                                                     20505                                                                              stam                                                                              20093                                                                   20508                                                                              jmp                                                                               20023                                                                   20511                                                                              nop                                                                        20512                                                                              nop                                                                        20513                                                                              1da                                                                               78                                                                      20515                                                                              stam                                                                              20094                                                                   20518                                                                              1da                                                                               134                                                                     20520                                                                              stam                                                                              20093                                                                   20523                                                                              jmp                                                                               20023                                                                   20526                                                                              tax                                                                        20527                                                                              tax                                                                        20528                                                                              1da                                                                               251                                                                     20530                                                                              stam                                                                              20020                                                                   20533                                                                              jmp                                                                               20054                                                                   20536                                                                              tax                                                                        20537                                                                              tax                                                                        20538                                                                              ida                                                                               78                                                                      20540                                                                              stam                                                                              20094                                                                   20543                                                                              1da                                                                               134                                                                     20545                                                                              stam                                                                              20093                                                                   20548                                                                              jmp                                                                               20000                                                                   20551                                                                              tax                                                                        20552                                                                              tax                                                                        __________________________________________________________________________ 

What is claimed is:
 1. In a pacemaker of the type having:(a) an atrial sense amplifier for producing atrial sense event signals (AS) in response to atrial depolarizations; (b) a ventricular sense amplifier for producing ventricular sense event signals (VS) in response to ventricular depolarizations; (c) an atrial pulse generator for producing atrial stimulating pulses in response to atrial pace event signals (AP); (d) a ventricular pulse generator for producing ventricular stimulating pulses in response to ventricular pace event signals (VP); (e) timing means for timing V-A escape intervals and for generating said atrial pace event signals at the conclusion of said V-A escape intervals and for timing A-V escape intervals and for generating said ventricular pace event signals at the conclusion of said A-V escape intervals, said V-A escape intervals initiated by the occurrence of said ventricular pace event signals or said ventricular sense event signals, said A-V escape intervals initiated by the occurrence of said atrial sense event signals or said atrial pace event signals, the improvement wherein; (f) said timing means includes means for dividing each said V-A escape interval into a first time interval and a subsequent second time interval, said timing means initiating timing of said A-V escape intervals in response to the occurrence of said atrial sense events during one of said second time intervals, but not in response to the occurrence of a said atrial sense event signal during one of said first time intervals, said timing means including means responsive to one of said atrial sense event signals occurring within one of said second time intervals to decrease the duration of a subsequent one of said V-A escape intervals and the duration of said first time interval therein.
 2. A pacemaker according to claim 1 wherein said timing means is further responsive to the absence of said atrial sense event signals occurring within one of said second time intervals to increase the duration of a subsequent one of said V-A escape intervals and the said first time interval therein.
 3. A pacemaker according to claim 1 or claim 2 wherein said timing means further comprises means for defining an inhibition interval beginning a predetermined time before the expiration of each said V-A escape interval, said timing means responsive to the occurrence of one of said atrial sense event signals which occurs in both the said first time interval and the said inhibition interval during one of said V-A escape intervals for preventing the generation of an atrial stimulating pulse at the expiration of said one of said V-A time intervals and for initiating timing of a said A-V escape interval at the expiration of said one of said V-A escape intervals.
 4. A pacemaker according to claim 1 wherein said means responsive to one of said atrial sense events occurring within one of said second time intervals to decrease the duration of a subsequent one of said V-A escape intervals comprises means for measuring the time between the occurrence of said one of said atrial sense event signals and the immediately previous ventricular pace event signal and means for setting the duration of said subsequent one of said V-A escape intervals as a function of the measurement performed by said measuring means.
 5. In a pacemaker of the type having:(a) an atrial sense amplifier for producing atrial sense event signals (AS) in response to atrial depolarizations; (b) a ventricular sense amplifier for producing ventricular sense event signals (VS) in response to ventricular depolarizations; (c) an atrial pulse generator for producing atrial stimulating pulses in response to atrial pace event signals (AP); (d) a ventricular pulse generator for producing ventricular stimulating pulses in response to ventricular pace event signals (VP); and (e) timing means for timing V-A escape intervals and for generating atrial pace event signals at the conclusion of said V-A escape intervals and for timing A-V escape intervals and for generating ventricular pace event signals at the conclusion of said A-V escape intervals, said V-A escape intervals initiated with the occurrence of said ventricular sense event signals or said ventricular pace event signals, said A-V escape intervals initiated with the occurrence of said atrial sense event signals or said atrial pace event signals, the improvement wherein; (f) said timing means further comprises means for dividing each said V-A escape interval into a first time interval and a subsequent second time interval, said timing means initiating timing of said A-V intervals in response to said atrial sense event signals occurring during said second time intervals, but not in response to said atrial sense event signals occurring during said first time intervals, said timing means further comprising means responsive to the occurrence of one of said atrial sense event signals during one of said second time intervals for decreasing the duration of a subsequent one of said first time intervals in a subsequent one of said V-A escape intervals.
 6. A pacemaker according to claim 5 wherein said timing means further comprises means responsive to the absence of said atrial sense event signals occurring within one of said second time intervals to increase the duration of a subsequent one of said first time intervals in a subsequent one of said V-A escape intervals.
 7. A pacemaker according to claim 5 or claim 6 wherein said timing means further comprises means for defining an inhibition interval beginning a predetermined time prior to the expiration of each said V-A escape interval and wherein said timing means includes means responsive to the occurrence of one of said atrial sense event signals which occurs during both the said first time interval and the said inhibition interval during one of said V-A escape intervals for preventing generating of an atrial pace event signal at the conclusion of said one of said V-A escape intervals and for initiating timing of said A-V escape interval at the expiration of said one of said V-A escape intervals.
 8. A pacemaker according to claim 1 or claim 5 wherein said means for dividing each said V-A escape interval into a first time interval and a second time interval comprises means for adjusting the durations of said first time interval and said second time interval such that the proportional relationship (SF) of the durations of said first and second time intervals remains constant from one of said V-A escape intervals to the next of said V-A escape intervals.
 9. In a pacemaker of the type having:(a) an atrial sense amplifier for producing atrial sense event signals (AS) in response to atrial depolarizations; (b) a ventricular pulse generator for producing ventricular stimulating pulses in response to ventricular pace event signals (VP); (c) timing means for timing A-V escape intervals following the occurrence of said atrial sense event signals and for generating said ventricular pace event signals at the conclusion of said A-V escape intervals, the improvement wherein; (d) said timing means further comprises means for determining first time intervals following said ventricular pace event signals and second time intervals following said first time intervals, said timing means initiating the timing of said A-V escape intervals in response to said atrial sense event signals occurring during said second time intervals, but not in response to said atrial sense event signals occurring during said first time intervals, said timing means further comprising means responsive to the occurrence of one of said atrial sense event signals within one of said second time intervals for decreasing the duration of a subsequent one of said first time intervals.
 10. A pacemaker according to claim 9 wherein said timing means further comprises means responsive to the absence of said atrial sense event signals occurring within one of said second time intervals to increase the duration of a subsequent one of said first time intervals.
 11. A pacemaker according to claim 5 or claim 9 wherein said means responsive to the occurrence of one of said atrial sense event signals during one of said second time intervals for decreasing the duration of a subsequent one of said first time intervals further comprises measuring means for measuring the time between the occurrence of said one of said atrial sense event signals and the immediately preceding one of said ventricular pace event signals, said timing means further comprising means for setting the duration of said subsequent one of said first time intervals as a function of the time measured by said measuring means.
 12. In a pacemaker of the type having:(a) an atrial sense amplifier for producing atrial sense event signals in response to atrial depolarizations; (b) a ventricular sense amplifier for producing ventricular sense event signals in response to ventricular depolarizations; (c) a ventricular pulse generator for producing ventricular stimulating pulses in response to ventricular pace event signals; (d) timing means for timing A-V escape intervals following the occurrences of said atrial sense event signals and for generating said ventricular pace event signals at the conclusion of each of said A-V escape intervals, said timing means further comprises means for generating a said ventricular pace event signal a predetermined time period following the generation of a previous said ventricular pace event signal, in the absence of said atrial sense event signals and said ventricular sense event signals, the improvement wherein; (e) said timing means further comprises means for determining a first time interval following each said ventricular pace event signal and a second time interval following each said first time interval, said timing means initiating the timing of said A-V escape intervals in response to said atrial sense event signals occurring during said second time intervals, but not in response to said atrial sense event signals occurring during said first time intervals, said timing means further comprising means responsive to the occurrence of one of said atrial sense event signals within one of said second time intervals for decreasing the duration of the said first time interval following a subsequent one of said ventricular pace event signals.
 13. A pacemaker according to claim 12 wherein said timing means further comprises means responsive to the lack of said atrial sense event signals during one of said time intervals for extending the duration of the same first time interval following a subsequent one of said ventricular pace event signals.
 14. A pacemaker according to claim 12 or claim 13 wherein said timing means further comprises means responsive to the occurrence of one of said atrial sense event signals during one of said second time intervals for decreasing the duration of a subsequent one of said predetermined time periods and the said first time interval therein and means responsive to the lack of said atrial sense event signals during one of said second time intervals for increasing the duration of a subsequent one of said predetermined time periods and the duration of said first time interval therein.
 15. A pacemaker according to claim 14 wherein said means responsive to the occurrence of one of said atrial sense event signals within one of said second time intervals for decreasing the duration of a subsequent one of said predetermined time periods comprises means for measuring the time between the occurrence of said one of said atrial sense event signals and the immediately preceding one of said ventricular pace event signals, said timing means further comprising means for setting the duration of said subsequent one of said predetermined time periods as a function of the measurement performed by said measuring means.
 16. A pacemaker according to claim 12 wherein said means responsive to the occurrence of one of said atrial sense event signals within one of said second time intervals for decreasing the duration of the said first time intervals following a subsequent one of said ventricular pace event signals, further comprises means for measuring the time between the occurrence of said one of said atrial sense event signals and the immediately preceding one of said ventricular pace event signals, said timing means further comprising means for setting the duration of the said first time interval following said subsequent one of said ventricular pace signals as a function of the time measured by said measuring means.
 17. In a pacemaker of the type having:(a) an atrial sense amplifier for producing atrial sense event signals (AS) in response to atrial depolarizations; (b) a ventricular sense amplifier for producing ventricular sense event signals (VS) in response to ventricular depolarizations; (c) an atrial pulse generator for producing atrial stimulating pulses in response to atrial pace event signals (AP); (d) a ventricular pulse generator for producing ventricular stimulating pulses in response to ventricular pace event signals (VP); (e) timing means for timing V-A escape intervals and for generating said atrial pace event signals at the conclusion of said V-A escape intervals and for timing A-V escape intervals and for generating said ventricular pace event signals at the conclusion of said A-V escape intervals, said V-A escape intervals initiated by the occurrence of said ventricular pace event signals or said ventricular sense event signals, said A-V escape intervals initiated by the occurrence of said atrial sense event signals or said atrial pace event signals, the improvement wherein; (f) said timing means includes means for dividing each said V-A escape interval into a first time interval and a second time interval, said timing means initiating timing of said A-V escape intervals in response to the occurrence of said artial sense event signals during said second time intervals but not in response to the occurrence of said atrial sense event signals during said first time intervals, said timing means further comprising means for determining an inhibition interval K3 beginning a predetermined time prior to the expiration of each said V-A interval and means responsive to the occurrence of one of said atrial sense event signals which occurs in both the said first time interval and the said inhibition interval K3 during one of said V-A escape intervals for preventing the generation of an atrial stimulating pulse at the expiration of said one of said V-A time intervals and for initiating timing of said A-V escape interval at the expiration of said one of said V-A escape intervals.
 18. A pacemaker according to claim 17 wherein said timing means further comprises means responsive to one of said atrial sense event signals occurring within one of said second time intervals to decrease the duration of a subsequent one of said V-A escape intervals.
 19. A pacemaker according to claim 17 wherein said timing means comprises means responsive to one of said atrial sense event signals occurring within one of said second time intervals to decrease the duration of a subsequent one of said first time intervals.
 20. A pacemaker according to claim 17 or claim 18 or claim 19 wherein said means responsive to the occurrence of one of said atrial sense event signals which occurs in both the said first time interval and the said inhibition interval K3 during one of said V-A escape intervals comprises means responsive to the occurrence of one of said atrial sense event signals during one of said time intervals for measuring an AS-AP interval which comprises the interval between the occurrence of said one of said atrial sense event signals and the expiration of said one of said V-A time intervals and for comparing said AS-AP interval to said inhibition interval K3 to determine whether said one of said atrial sense event signals also occurs within said inhibition interval K3.
 21. A pacemaker according to claim 1 or claim 6 or claim 10 or claim 12 or claim 17 wherein said timing means further comprises means for determining an absolute refractory period following each said ventricular sense event signal and each said ventricular pace event signal during which said timing means is nonresponsive to said atrial event signals. 